1. Field of the Invention
The present invention relates generally to a printing blanket and a method of manufacturing the same. More particularly, the present invention relates to a printing blanket having excellent properties in respect of transference of a printing ink onto a printing medium such as a paper or the like as well as releasability of the printing medium from the printing blanket. Further, the present invention relates to a method of producing a printing blanket of the foregoing type.
2. Description of the Related Art
To facilitate understanding of the present invention, a conventional offset printing press will briefly be described below with reference to FIG. 1 and FIG. 2.
As shown in FIG. 1, the offset printing press is basically constructed by a printing plate drum 1 having a printing plate serving as an original wound thereabout, a printing blanket drum 2 adapted to be rotated in operative association with the printing drum 1, and a pressure drum for squeezing a printing paper 3 against the printing blanket drum 2 from below. A printing blanket 5 is wound around the printing blanket drum 2.
In operation, the printing plate wound around the printing plate drum 1 is coated with an ink so that an image formed by the coated ink is once transferred onto the printing blanket 5 wound around the printing blanket drum 2 and the transferred ink image is then printed on the printing paper 3.
As shown in FIG. 2, the printing blanket 5 used for the printing press as mentioned above is generally constructed such that a compressible layer 52 molded of an elastic material is placed on a base fabric 51 and a surface layer 53 is additionally placed on the compressible layer 52 so that the printing paper 3 comes in contact with the surface layer 53 during a printing operation.
A problem appearing when the printing blanket 5 is used for performing printing operations is such that the surface layer 53 is chemically corroded by an oil based printing ink. For this reason, a low-graded nitrile butadiene rubber having excellent printing properties in respect of transferance of an ink onto a printing medium such as a paper or the like but having inferiority in respect of resistibility against an oil has not been employed for the surface layer 53 but an intermediate/high-graded nitrile butadiene rubber has been hitherto employed for the same. In practice, the intermediate/high-graded nitrile butadiene rubber has an excellent oil resistance. However, it has drawbacks associated with a high critical surface tension of 30 dyn/cm, poor ink transferability and poor paper releasability.
To improve the oil resistibility of the printing blanket, a proposal has been made as to a printing blanket which is coated with, e.g., tetrafluorethylene-propylene copolymer to be later cured, as disclosed in Japanese Examined Publication Patent (Kokoku) NO. 55-38682.
According to this prior invention, basically, the surface layer of the printing blanket is coated with a film having an excellent oil resistibility. However, since this oil resistant film has a very small thickness, there unavoidably appears a phenomenon that the film is readily peeled off from a base fabric. Another problem is that factors of ink transferability and paper releasability should additionally be taken into account for the purpose of manufacturing a printing blanket.
In addition, to improve the ink transferability while an adhering power of the ink to the surface layer is reduced, another proposal has been made as to a printing blanket including a surface layer having a coated film of which critical surface tension is controllably determined within the range of 18 to 25 dyn/cm, as disclosed in Japanese Examined Publication Patent (Kokoku) NO. 56-36077.
Recently, it has been known in the art that receivability of an ink on a printing blanket and transferability of the same on a printing medium such as a paper or the like do not depend merely on the critical surface tension of a thin film on the surface layer of the printing blanket.
Specifically, according to the latter prior invention, silicone, trifluorethylene, polyfluorovinylidene, polypropylene or the like are employed as a material for a layer to be coated. However, it has been known in the art that each of the aforementioned synthetic resins exhibits poor ink receivability even when a critical surface tension less than 18 dyn/cm appears on the coated thin film on the surface layer of the printing blanket.
On the other hand, it has been known in the art that e.g., a fluororubber exhibits excellent ink receivability and transferability even when a critical surface tension less than 18 dyn/cm appears on the film on the surface layer of the printing blanket. In other words, employment of the fluororubber makes it possible to manufacture a printing blanket having a low critical surface tension (i.e., exhibiting excellent ink transferability and paper releasability) as well as excellent ink receivability. In the aforementioned circumstances, there have been hitherto raised from users many requests for providing the printing blanket as mentioned above.